The olfactory system plays an important role in an organism’s ability to grasp its environment. One of the most important regions of the brain responsible for olfaction is the olfactory bulb. The olfactory bulb processes molecular information from specific chemoreceptors called olfactory receptors and interprets it as a scent. This organ is responsible for an organism’s ability to identify smell, also known as the olfactory sense. In addition to being an important sensory system for survival for many organisms, the sense of smell is thought to be most associated with memory due to the olfactory bulb’s location in the brain.
History and Function
Richard Axel (1946—) and Linda Buck (1947—) were awarded the 2004 Nobel Prize in Medicine for their work with olfactory receptors (Keeley, 2004). During their study, they discovered the genes responsible for olfactory receptors. They also found that olfactory receptors have high specificity to vaporizable molecules, also known as aromatics.
These receptors have axons that extend all the way to the olfactory bulb. Because these receptors are highly specific to their binding agent, they can be classified into families. Each family of receptors converge to a single point, creating a bundle of nerve endings called a glomerulus. The main olfactory bulb is home to these glomeruli, and it is where scents are identified and interpreted.
An activated glomerulus enables the olfactory bulb to decipher the scent specific to those receptors. Because each glomerulus is associated with a single type of molecule, scientists have mapped the main olfactory bulb to locate where different odors are deciphered within it.
The accessory olfactory bulb is part of the olfactory bulb and has been attributed to mating and social behaviors in organisms. It responds to pheromones in addition to ordinary aromatics. Although the existence of this structure has been largely identified in mice and rodents, it is thought to be absent in adult humans. Scientists found an accessory olfactory bulb in human embryo, but later stages show the structure simplifies and eventually disappears as the embryo develops (Shepherd, 2010).
Anatomy and Physiology
The olfactory bulb is located on the inferior part of the forebrain. Olfactory receptors bind to specific molecules in the air. Correct molecules will trigger a cascade of action potentials, serving as a signal that is then relayed to the specific glomerulus of the receptors. This information is then relayed to other parts of the brain, such as the hippocampus and amygdala. The hippocampus is responsible for memory formation whereas the amygdala is the primary processor of emotions. The olfactory bulb is directly linked to both of these structures. This is thought to be the reason for the strong correlation between scents and memories.
The vomeronasal system is a part of the auxiliary olfactory system that is largely governed by the accessory olfactory bulb. Although adult humans largely lose the accessory olfactory bulb during development, it plays a huge role in other mammals in choosing partners (Shepherd, 2010). Pheromones are airborne hormones detectable by the vomeronasal system, which contributes to the chemistry of attraction during mating season in many animals. The accessory olfactory bulb is located in the nasal passage and is directly connected to the amygdala. Due to the absence of this structure in adult humans, it is unclear whether or not the vomeronasal system exists outside of animals.
Disease and Disorders
Disease and disorders in the olfactory bulb cause an inability to distinguish scents. The degree of the defect depends on the severity. Many mechanisms can cause a defect, including facial trauma and neurodegeneration (Attems et al., 2014). Due to the location of the olfactory bulb, facial trauma can cause damage to the receptors as well as the bulb itself. Neurodegenerative diseases such as Parkinson’s and Alzheimer’s are also known to affect the olfactory bulb as the disease progresses. Any damage to the olfactory bulb and its associated receptors will cause a deficit in the organism’s ability to process scents from its surrounding. This may affect appetite as well because the smell of food is known to contribute to gustation, or sense of taste.
See also: Anosmia; Axel, Richard; Bowman’s Glands; Buck, Linda; Olfactory System
Attems, Johannes, Lauren Walker, & Kurt A. Jellinger. (2014). Olfactory bulb involvement in neurodegenerative diseases. Acta Neuropathologica, 127(4), 459—475. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/24554308
Keeley, Jim. (2004). Richard Axel and Linda Buck awarded 2004 Nobel Prize in Physiology or Medicine. Howard Hughes Medical Institution. Retrieved from http://www.hhmi.org/news/richard-axel-and-linda-buck-awarded-2004-nobel-prize-physiology-or-medicine
Shepherd, Gordon M. (2010). New perspectives on olfactory processing. In A. Menini (Ed.), The Neurobiology of Olfaction (Chap. 16). Boca Raton, FL: CRC Press/Taylor & Francis. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK55977/